Search Engine Detection: plugin hook

[robocoder]

From #1694.

a hook so plugins can implement their own search engine detection as there are requests for sites to be added that don't quite fit the traditional definition of a search engine.

[robocoder]

(In [3421]) refs #1877 - add hook

[robocoder]

(In [3422]) refs #1877 - pass referrer to plugin

[robocoder]

ToDo: changes to plugins/Referers/functions.php to support custom search engines, e.g., path to image, search engine name to URL, etc.

[sgiehl]

In 2e215ff: refs #1877 added hooks to add search engines and social networks; some code improvements